Immunodeficiency associated with aging is thought to contribute to the high incidence of infection and neoplastic disease in the geriatric population. Descriptive and functional abnormalities have been observed in both cellular and humoral immune responses, however, the greatest changes are observed in the former. Thymic involution, which gradually occurs after sexual maturation in mammalian species, precedes the observed defects in cell mediated immunity and is considered by many to be causally related to immune senescence. In the studies planned we intend to further characterize immune function in normal elderly volunteers by measuring specific antibody production in vivo and in vitro after tetanus toxoid' booster' immunization. The potential restorative effects of various thymic hormones will also be evaluated in vitro. Additionally, in mice, we plan to assess similar immune functions with aging and test the reconstitutive capacity of syngeneic or allogeneic thymic implants. The overall goals are to provide basic in vitro and in vivo research support for future clinical trials of thymic hormones or cultured thymus transplants as an effort to reconstitute immune deficient elderly people.